Particle detectors are often used to warn of the presence of smoke emanating from a potential or incipient fire.
Smoke detectors operate in a variety of environments and include, for example, office environments, factories and manufacturing plant, power stations and clean rooms. Each of which has different levels of background particulate material. In some environments the concentration of background particulate matter will change from time to time.
A problem can be encountered with such smoke detection apparatus if they are continuously exposed to the relatively high levels of background pollution in the air that can exist in some environments. A large-scale example in recent years has been the high levels of smoke pollution often present in regions of Asia, which is largely attributable to the burning of brown coal.
Background pollution can cause contamination of components within the detector leading to premature failure, for example due to clogging of air paths or changes in the optical properties of the critical components within the detection chamber.
Scattering light detectors include a light source arranged to project a beam across the detection chamber. A photoelectric sensor is arranged so that its field of view is traversed by a portion of the beam. The photoelectric sensor receives light scattered from the beam due to the presence of particles in the detection chamber. Over time dust and debris can accumulate on surfaces within the detection chamber and reflect light towards the photoelectric sensor thereby providing a false indication of particles in the detection chamber. Dust and debris may also settle on the light source and/or the photoelectric sensor thereby obscuring the transmission and receipt of light and reducing the sensitivity of the detector.
One approach to addressing these problems involves the use of an ‘air barrier’. An air barrier is created by directing one or more streams of clean air into the detection chamber to flow over the critical components, such as the light source, the photoelectric sensor and walls within the field of view of the photoelectric sensor, to prevent dust and debris accumulating thereon.
Aspirated smoke detectors employ a fan, known as an aspirator, to draw air to be studied through the detection chamber. The air to be studied enters the chamber via an inlet(s). A desirable implementation of the air barrier concept employs a filter(s) to create the clean air. The filter is arranged in parallel to the inlet, whereby the clean air is drawn through the filter and into the detection chamber by the aspirator. A common stream of air, e.g. from a network of pipes, may be divided into two portions—one portion being filtered to create the clean air, and the other portion entering the chamber to be studied.
Another approach to addressing the problems associated with the accumulation of dust and debris in the detection chamber is to obtain a measurement associated with light reflected from the accumulated dust and debris, known as ‘background light’, and to adjust the detection criteria applied to the signal received from the photoelectric sensor in response to the background light. One approach to obtaining a measure of background light involves the use of a second photoelectric sensor within the detection chamber. The second photoelectric sensor is arranged so that its field of view does not include the beam. Signals from the second photoelectric sensor are thereby indicative of light reflected within the detection chamber rather than light scattered directly from the beam.
The abstract of Japanese patent application 59192940 entitled “Smoke Meter with Purging Device” describes filling a measuring device with clean air and measuring opaqueness in the clean atmosphere to perform calibration. The described device includes a dedicated blower to supply clean air to the detection chamber. A valve controlled by depressible switches is used to close the intake tube to halt the flow of discharge gases to the detection chamber prior to the purging operation.
New Zealand patent 250497 is concerned with preventing fire suppression measures being activated in response to false alarms. It describes an operating syntax applicable to aspirated smoke detectors. When an alarm condition is detected the chamber is purged with dean air and a background ‘smoke’ signal is measured. If the background reading does not fall below a predetermined threshold a detector fault is indicated. If the background ‘smoke’ falls below the predetermined threshold, the system waits for the detected smoke level to rise above a further threshold before triggering the fire suppression systems.
Other attempts to overcome the problems associated with operating particle detectors in polluted environments have included dust filters placed in the airstream. Dust filters have been used to filter out particles not associated with the smoke to be detected. Smoke particles may occur in a variety of sizes depending on the fuel used and combustion conditions, and the filter type is chosen according to the type of dust particles expected and the type of smoke to be detected.
As conventional dust filters clog they begin to remove more particles from the air and eventually begin filtering out smoke particles (or other small particles of interest). This may be due to effective pore size of the filter being reduced as more particles clog the filter. Certain types of filters, particularly foam fitters, can start removing smoke particles before the pressure drop across, or flow rate through, the filter changes appreciably. The result is that the filter may be removing an unknown proportion of smoke long before clogging of the filter is detectable using pressure and/or flow measuring devices.
In some situations attempts have been made to condition the air sample prior to its introduction into the smoke detector e.g. by diluting the sample flow with clean air. The object of such dilution is to deliver to the detection chamber a sample flow with an unchanged particle distribution, but with a lower particle concentration than the original sample flow. Whilst such dilution arrangements go some way to addressing the problems associated with operating in a polluted environment, the lower particle concentration reduces the sensitivity and accuracy of the detector.
Dilution presents problems for air sampling smoke detectors that use a pipe network to draw air from a space being monitored, in that the introduction of the diluent air flow into the flow entering the detector reduces the amount of sample air drawn from the region being monitored. This causes an increase in the time taken for the sample air to travel from the region being monitored to the smoke detector, referred to as “transparent time”, and consequently increases detection time.
The applicant has proposed, in their international patent application WO 2007/095675, an arrangement wherein a first portion of a sample flow is filtered through a HEPA (high efficiency particulate air) filter. The HEPA filter removes substantially all particles from the first portion of the sample flow to form clean air. The clean air is used to dilute a second, unfiltered, portion of the sample flow. The diluted sample flow is in turn carried to a detection region. This arrangement effectively addresses the transit time problem associated with other dilution arrangements and has a desirable ‘fail safe’ operation in that if the filter is allowed to clog to an extent that it becomes more restrictive to flow, the detection region will see an increase, rather than a decrease, in the concentration of particles. The extent of dilution may also vary due to environmental factors such as temperature and humidity. The changing dilution ratio reduces the accuracy of the associated smoke detector.
Despite these various advances in the art, the known filtration arrangements and dilution arrangements result in a reduced concentration of particles arriving at the detection region and can reduce the sensitivity and accuracy of the particle detector. It is of course desirable that a smoke detector should be sensitive and accurate. It is also desirable that if a filter is used, that it's condition, and in particular whether it has clogged to an extent that it is removing particles of interest (e.g. smoke particles), is known.
Objects of the invention include providing improved particle detection, an improved particle detector and components therefor, or at least providing alternatives for those concerned with particle detection.
Reference to any prior art in the specification is not, and should not be taken as, an acknowledgment or any form of suggestion that this prior art forms part of the common general knowledge in Australia or any other jurisdiction or that this prior art could reasonably be expected to be ascertained, understood and regarded as relevant by a person skilled in the art.